Reflections from the objective lens or other reflective surfaces of an optical system (glint) have long been a problem, especially in a battlefield environment. These reflections turn out to also be a problem with wide-angle field-of-view (FOV) optics such as night vision goggles. This is especially so when operating in an environment where relatively bright ambient sources such as street lights are present, or in situations where the enemy also has night vision equipment and thus can see reflections of moon or starlight from an objective lens or reflective filter.
An existing method of reducing or eliminating such reflections is to put a honeycomb grid of tubes in front of the objective lens (as is described in U.S. Pat. No. 4,929,055, which is fully incorporated herein by reference). The tubes in these devices have walls that are parallel to the optical axis of the device to which it is fitted.
This technique, however, is not an effective solution with wide angle FOV devices, since if the length-to-width ratio of the tubes which make up the honeycomb of parallel-walled tubes is shallow enough not to vignette the view through the optic, then the tubes are not deep enough to give affective glint protection. This means that in a battlefield situation, wide-angle FOV optical devices are vulnerable to being detected by an envoy, and thus dangerous to use.
Accordingly, it is highly desirable, if not necessary, to devise other techniques for substantially preventing reflections from the reflecting surfaces of wide-angle FOV optical devices.
As can be seen in FIG. 1, a reflective element 1 of an optical device 2 can reflect light rays 5 from a light source 3 to an observer 4. The Observer 4 includes sophisticated light detection systems possibly operating in the infrared and ultraviolet spectrums as well as human or animal observers.
An existing method of hiding such reflections is shown in FIG. 2. where a honeycomb of parallel-walled tubes 6 is placed in front of the optical device 2. The walls of the tubes are parallel to the optical axis of the device to which it is fitted. This collection of tubes 6 prevents light from a source 3 from reflecting to an observer 4.
As shown in FIG. 3, the length-to-width ratio of the tubes 6 that make up the honeycomb cannot exceed the length-to-width ratio of the FOV 13 of the optical device to which it is fitted. In this way, the anti-reflection shield does not restrict field of view seen through the optical device.
As shown in FIG. 4, an example of this would be the U.S. Army's PVS-7 night vision goggles, which have a FOV 13 of 40°. If one were to use the existing method of reflection protection, the length-to-width ratio of the deepest (longest) tubes 6 that could be used in a conventional anti-reflection shield are 1:1.38. This is not deep enough to give good glint protection. If deeper tubes are used, they would intrude on the FOV and vignette the image seen through the device, as illustrated in FIG. 5.
The problem has been how to get tubes long enough to provide effective glint protection without vignetting the view through the optic.